In a spread spectrum system of interest to these teachings a pilot channel signal is transmitted from a base station or base site for reception by mobile terminals within an area served by the base station. The mobile terminal could be a cellular telephone or a PCS terminal. The received pilot signal is used by the mobile terminal in a searcher function for synchronization and system acquisition purposes, for allocating despreaders or correlators, as well as for mobile station demodulation processing of the data bearing channels. However, the pilot signal suffers from the same radio channel fading processes as does the normally transmitted signal. Due to the fading processes the signal to noise ratio (SNR) of the received pilot signal varies as a function of time and, as a result, the use of the pilot signal for its intended purposes can be impaired.
The searcher is generally used for synchronization acquisition and channel set maintenance. The synchronization acquisition procedure is intended to synchronize a local pseudo-noise (PN) generator to CDMA system time. During the set maintenance procedure the searcher identifies the multipath profile of the channel and calculates the pilot channel energy per chip to total received power spectral density ratio (Ec/Io).
A problem arises in this regard, as the searcher sample size may be, for example, 2048 chips (single dwell) or more. In this case it can be shown that several seconds may be required to compute all of the correlations and, thus, the acquisition time can be dominated by the correlation calculations.
In accordance with a conventional searcher operational procedure, a data processor of the mobile terminal may use the power measurement capabilities of an automatic gain control (AGC) block in order to determine if data stored into searcher memory (also referred to as a buffer) is subject to any fading (or other adverse) channel conditions. However, there are problems with this approach, as the response time and other characteristics of the AGC may not be optimum for use by the searcher function.
The following commonly assigned U.S. Patents are referenced for teaching various aspects of spread spectrum searcher synchronization and acquisition: U.S. Pat. No. 5,440,597, issued Aug. 8, 1995, “Double Dwell Maximum Likelihood Acquisition System With Continuous Decision Making for CDMA and Direct Spread Spectrum System”, by S. Chung et al. and U.S. Pat. No. 5,642,377, issued Jun. 24, 1997, “Serial Search Acquisition System with Adaptive Threshold and Optimal Decision for Spread Spectrum Systems, by S. Chung et al. Reference can also be made to U.S. Pat. No. 6,078,570, issued Jun. 20, 2000, “Mobile Assisted Hard Hand-Off for a Code Division Multiple Access (CDMA) System”, by S. Czaja et al., for teaching the use of pilot channel signal strength measurements and automatic gain control (AGC) in the context of mobile terminal hand-offs. The disclosures of these commonly assigned U.S. patents are incorporated by reference herein in their entireties.